As electronic devices are getting smaller and lighter, demands for downsizing parts applied to printed circuit boards are increasing. In addition, a high degree in design freedom and excellent flexibility are required in order to be suitable for high-density wiring. Moreover, electronic devices are operated at higher performance and higher speed, resulting in generation of noise due to higher frequency. Therefore, the focus of developing EMI shielding materials has become even more important.
Shielding methods of these electronic devices may be classified into a shielding film type, a type where silver paste is coated on a printed circuit board, and a sputtering type where metal particles are attached on a printed circuit board. Considering process workability, reliability, and higher performance, a film-type shielding method is advantageous.
As an example of an EMI shielding film according to the related art, Korean Patent Laid-Open Publication No. 10-2008-0015447 discloses an EMI shielding film that includes a separating film; a release layer; a double-layer structured cover film consisting of a hard layer and a soft layer; a metal layer; and a conductive adhesive layer.
However, this EMI shielding film is easily torn at the time of FPCB processing because the cover film is brittle, resulting in delamination of the cover film and the metal layer from each other. As a result, a plating solution infiltrates into the EMI shielding layer, thereby causing problems in chemical resistance of the EMI shielding film.
Furthermore, this EMI shielding film is easily released from a flexible printed circuit board (FPCB) at the time of prior lamination with the FPCB since the conductive adhesive agent has low adhesive strength at room temperature.
As another example of an EMI shielding film according to the related art, Korean Patent Laid-Open Publication No. 10-2007-0110369 discloses an EMI shielding film that includes a reinforcing film; a slight adhesive layer; a base film; and a curable conductive adhesive layer containing a poly urethane-poly urea resin, an epoxy resin having two or more epoxy groups, and a conductive filler.
This EMI shielding film is designed such that a shielding performance is given to the conductive adhesive layer, since there are no interlayer metal layers and the conductive adhesive agent is prepared in an isotropic type from an anisotropic type. However, when this EMI shielding film is long flexed, a film formed by metal particles is easily cut, and thus the EMI shielding performance cannot be maintained.